The subject matter of the present invention includes a method of making a float glass that is convertible or transformable into a glass ceramic and the float glass made by the method.
Methods of making float glass are well known. Molten glass is supplied to a metal bath, which usually comprises tin or a tin alloy, is formed into a glass sheet of a predetermined thickness, is guided though zones of different temperature on the metal bath, is cooled on it, and finally continuously drawn off from the metal bath. The glass mass spreads out on the metal bath surface when it is poured on the float bath, until it has reached an equilibrium thickness, which is determined by the density of the glass mass, the density of the tin, and the properties of the boundary surface between the tin and the glass. The thickness of the glass mass on the metal bath surface usually reaches about 7 mm. When thin glass is to be produced, the glass on the melt is drawn. The glass is drawn with the assistance of top rollers, which are placed at different positions on the sheet, in order to reduce the sheet thickness and increase the sheet width. The top rollers are cooled rollers, which are driven with a definite and adjustable rotation speed. By using several top roller pairs in suitable rotation speed stages and angular positions, the thickness of the glass sheet is successfully reduced, without a too great reduction of the sheet width, or even an increase in the sheet width.
To produce good quality glass the temperature of the glass sheet is reduced with a relatively constant and comparatively small cooling rate of about 20 to under 30° C. min−1 over a large temperature range of about 200° C. from about 1150° C. to 900° C. A careful temperature control in this temperature range is indispensable. This careful cooling process is required to minimize thickness variations and the fine waviness.
When this process is performed with crystallizable glass compositions, one usually obtains results, which are not sufficient to satisfy the elevated requirements. In the temperature range, in which the glass sheet is processed with a comparatively small cooling rate for the purpose of drawing the glass sheet, crystallization is already occurring, so that the later ceramicizing of the glass, i.e. its conversion into a glass ceramic, in which the glass is first held at a precisely defined temperature for an exact predetermined time interval, which allows crystals to grow after that at a higher temperature, is negatively influenced by the crystals formed during the drawing of the glass sheet in an undesirable manner.
Crystal nuclei or seeds formed during the drawing stage form two types of faults. The first type is a surface defect, which arises by interaction between the glass and the float bath and/or the float bath atmosphere. The second type is a defect within the glass, e.g. a platinum particle from the structural parts of the bath. Crystals can form at these defect locations because of the time required for drawing of the glass sheet.
Two different starting points for solution of this problem are found in the prior art. These two approaches are described in the following paragraphs.
According to U.S. Pat. No. 3,804,608 the glass is rapidly cooled immediately after formation to the crystallization temperature and held there for a comparatively longer time. This process has three disadvantages: first it may be performed only with specially selected glass, second flat glass can only be made with the equilibrium thickness, and third, above all, no definite neat crystal seed formation occurs in the process, since the glass is cooled to a crystal seed formation temperature, which is below the crystal growth temperature.
This latter disadvantage is avoided by a method described in U.S. Pat. No. 3,809,543 (=DE 22 07 727) of the same applicants as the foregoing US patent, in which the glass sheet is rapidly cooled to a temperature below the crystal nuclei formation temperature. Subsequently the temperature is raised to a nuclei formation temperature and then after that still further to a crystallization temperature. Also with this latter method only flat glass or float glass with an equilibrium thickness may be made.
Generally both these methods have the disadvantages of all coupled processes in comparison to uncoupled processes. The methods for glass ceramic production by ceramicizing the sheet have not been put into practice because of the above-described disadvantages.
An entirely different method is described in DE 100 17 701 C2. In that patent document a crystallizable glass is described, which may be drawn to a thickness under the equilibrium thickness and which does not have undesirable crystal nuclei on the underside of the glass sheet, which are formed during the drawing process.
However it has been shown that isolated defects in the form of crystallites are still formed within the glass during the drawing of the glass sheet to a thickness under the equilibrium thickness because of the slow cooling on the float bath that occurs with all glasses. This leads to waste. The cause of this undesirable crystal nucleation during drawing is not known. Possibly it is due to microscopic non-uniformities in the glass mass.